1. Field of the Invention
The present invention relates to a beam recorder for forming an image on a record medium by a beam.
2. Related Background Art
In a laser beam printer which turns on and off a laser beam in accordance with an image signal and forms an image by an electro-photographic system, two methods have been proposed with respect to the turn on/off of the laser beam and the image to be recorded.
One is so-called image scan system (negative development system) in which the laser beam is turned on at points corresponding to black pixels of the image and turned off at points corresponding to white pixels.
The other is so-called background scan system (positive development system) in which the laser beam is turned off at the points corresponding to the black pixels and turned on at the points corresponding to the white pixels.
In the image scan system and background scan system, the charging polarities of toner used to visualize an electrostatic latent image in a development process are opposite to each other.
FIG. 10 shows a principle of an image forming unit of a conventional laser beam printer. An image signal is applied to a laser beam source 21 and a laser beam is turned on and off in accordance with the image signal. The laser beam emitted by the laser beam source 21 passes through a collimater lens 20 and is collimated, and then focused to a predetermined beam diameter, directed to a rotating polygon mirror 22 which rotates at a constant speed, and is scanned over a surface of a photoconductor 12. A light reflected by the rotating polygon mirror 22 is corrected for light path by a correcting optical system 23 (f.theta. lens) and then focused onto the photoconductor 12. The laser beam is scanned on the photoconductor 12 in a direction of an arrow Hs one for each mirror plane as the rotating polygon mirror 22 rotates. Since the photoconductor 12 which has been uniformly charged by a charger (not shown) rotates at a constant speed in a direction shown by an arrow 26, the laser beam is two-dimensionally scanned so that an electrostatic latent image is formed on the photoconductor 12 in accordance with the turn-on and turn-off of the laser beam.
A portion of the scan light is reflected by a reflection mirror 24 provided at a predetermined position ahead of the scan area of the photoconductor 12 by the laser beam, directed to a photo-sensor 25 and converted to an electrical signal. This signal is used as a horizontal synchronization signal.
The electrostatic latent image thus formed on the photoconductor 12 is developed and transferred to a record sheet to record an image in a known electrophotographic method.
In recent years, most of the images printed by such a laser beam printers are document images. In the document image, the number of white pixels is usually much more than the number of black pixels.
In the image scan system, the laser beam is turned on only when the black pixels are to be formed. Accordingly, when the document image is to be outputted, an accumulated turn-on time of the laser beam is short and hence the image scan system is advantageous over the background scan system when a semiconductor laser having a short accumulated turn-on lifetime is used as a laser beam source. On the other hand, when a solid black image comprising continuous black pixels is to be printed, a spacing between scan lines slightly varies due to a plane angle precision of the planes of the rotating polygon mirror and a mechanical vibration thereof and a stripe pattern appears along the direction of movement of the photoconductor.
On the other hand, in the background scan system, the laser beam is not turned on at the black pixel areas and hence a uniform black area is printed.
There is also a difference between the image scan system and the background scan system when a fine line in the order of one pixel width is to be reproduced. This is due to the fact that an energy of a laser beam spot focused on the photoconductor may be considered to have a two-dimension Gauss distribution, and latent image formed on the photoconductor by such spots and a development characteristic.
The reproduction of the fine line by the respective systems is explained with reference to FIGS. 3A and 3B and FIGS. 4A and 4B.
FIGS. 3A and 3B illustrate the formation of the fine line by the image scan system. In FIG. 3A, a black line of one line width is to be formed, and in FIG. 3B, a white line of one line width is to be formed. Each circle corresponds to one pixel, and (white circle) indicates that the laser beam is not turned on and (hatched circle) indicates that the laser beam is turned on. An electrostatic latent image formed on the photoconductor has a Gauss energy distribution of laser spot as shown by 1 and 2 of FIGS. 3A and 3B. Thus, the energy spreads to the periphery and the black pixel area after the development is wider than one pixel area because of the development characteristic of the electrophotography. P.sub.1 and P.sub.2 represent development threshold levels of the electrostatic latent image. Thus, the black fine line is thickened while the white fin line is narrowed. Namely, the width l.sub.1 of the black fine line is wider than one pixel width and the width l.sub.2 of the white fine line is narrower than one pixel width.
As a result, when a complex character such as a Kanji character of a small size is to be printed, the character is distorted by the thickening of the black fine line and the narrowing of the white fine line. When the beam spot diameter is increased or the development threshold level is lowered so that the adjacent black pixels overlap in order prevent ununiformity in the solid black due to the limitation on the precision of the scanning optical system, the above trend is more and more accelerated, and in an extreme case, the white fine line of one pixel width disappears.
On the other hand, in the background scan system, the laser turn-on and turn-off are opposite to those of the image scan system. Thus, when a black fine line is to be reproduced by the turn-off of the laser, the black fine line is narrowed as shown in FIG. 4A, and when a white fine line is to be reproduced by the trun-on of the laser, the white fine line is widened. Namely, the width l.sub.1 of the black fine line is narrower than one pixel width and the width l.sub.2 of the white fine line is wider than one pixel width. As a result, a complex character of a small size is reproduced thinly, and the black fine line of one line width may not appear depending on the spot diameter of the laser beam and the development characteristic.
In the prior art image scan system, the black fine line is thickened and the white fine line is narrowed so that the reproduced image tends to be distorted. In the background scan system, the black fine line is narrowed and the white fine line is widened so that the reproduced image tends to be thinned.
Methods for obtaining a fine line of a proper width or a proper image are disclosed in U.S. Pat. Nos. 4,387,983; 4,476,474; and 4,517,579 assigned to the assignee of the present invention, but further improvement of those methods has been sought.